The present invention relates to optical reader devices, such as bar code readers, and is directed more particularly to an optical reader having improved exposure control means.
Optical readers, such as bar code readers, have become widely accepted and used in many fields because of their proven ability to read data from optically encoded indicia, such as bar code symbols. Such readers are not only able to read optically encoded data more quickly than human beings, they are able to read it more accurately and consistently.
In spite of their widespread use and acceptance, optical readers have limitations that can prevent them from being used under all of the conditions in which their use would be desirable. One of these limitations is that the photosensitive image sensing array thereof can be so underexposed under low light conditions that the output thereof is too dark to be readily decoded. Conversely, photosensitive image sensing array can be so overexposed under bright light conditions that the output thereof is too bright to be readily decoded. This is because, under both of these conditions, the output signal of the array provides a low contrast between the white and black elements of the indicia and because such low contrast results in poor signal-to-noise ratios.
Another of these limitations is that optical readers are often restricted to operation with a depth of field that is relatively shallow. In other words, optical readers may fail to produce a readily decodable output when the distance between the reader and its target indicia is too great. This is in part because large distances between the reader and the indicia decrease the total light intensity at the indicia and thereby tend to underexpose the readers photosensitive image sensing array. This limitation is particularly troublesome in the case of readers which rely on built-in light sources, such as LEDs, rather than on ambient light levels, to provide the illumination necessary for accurate reading.
Prior to the present invention, the above-discussed limitations have been dealt with in a variety of different ways. One of these is to provide the reader with automatic gain control (AGC) circuitry for increasing or decreasing the gain or loss applied to signals produced by the photosensitive array as necessary to cause those signals to have a predetermined standardized value. One example of a reader having such AGC circuitry is described in U.S. Pat. No. 4,528,444 (Hara, et al.).
Another approach to overcoming the above-discussed limitations is to provide the reader with exposure control circuitry for increasing or decreasing the time period during which the photosensitive sensing array is exposed. Because such arrays produce outputs that are dependent on the integral of the illuminating light intensity as a function of time, changes in the exposure time of the array can be used to increase or decrease the magnitude of the output signal as necessary to cause those signals to have predetermined standardized values. An example of a reader having exposure having exposure control circuitry of this type is described in U.S. Pat. No. 4,538,060 (Sakai, et al.).
Still other approaches to overcoming the above-discussed limitations include providing illumination control circuitry for controllably increasing and decreasing the amount of light which the reader directs at the indicia to be read, and distance indicating circuitry that produces a visual distance indication that allows a user to move the reader closer to or further from the target indicia. An example of a reader having circuitry of the former type is described in U.S. Pat. No. 4,818,847 (Hara, et al.).
While the above-described approaches to exposure control improve the performance of the readers with which they are used, they all have deficiencies which limit their usefulness or cause them to make inefficient use of reader circuitry or program space. A frequently encountered one of these deficiencies is that they operate continuously, always seeking to establish a precise, optimum exposure value. Such continuous efforts are inefficient because the benefits which result from their use become insignificant as the optimum exposure value is approached. As a result, a reader can devote large amounts of time and/or program space to producing only marginal improvements in reader performance.
One way of dealing with this inefficiency is to have the exposure control function performed by special purpose hardware, thereby effectively off-loading the burden of exposure control from the readers' programmable control circuitry. This off-loading can, however, increase the cost of the reader either by increasing its parts count or by requiring the use of a sophisticated or "smart" image sensor which has a built-in exposure control function.
Another deficiency of known exposure control circuits is that they can take a long time, i.e., many scans, to reach an acceptable exposure time value. This is particularly true in readers which always use the same initial exposure value, and which converge on their final exposure value in increments that are kept small in order to avoid overshooting or oscillating about that value.
Thus, a need has existed for an exposure control circuit and method which is not subject to the above-described deficiencies.